1. Field of the Invention
The present invention relates to an exposure apparatus, and more particularly to an exposure apparatus equipped with an interferometer for measuring a wavefront aberration of a projection optical system for projecting a pattern formed on a mask onto an object to be exposed, and an exposure method therefor.
2. Description of the Related Art
Conventionally, in manufacturing a semiconductor device such as an IC or an LSI, an image pickup device such as a CCD, a display device such as a liquid crystal panel, or a device such as a magnetic head in a photolithography process, there has been used a projection exposure apparatus that transfers a pattern formed on a mask (reticule) onto an object to be exposed. The projection exposure apparatus is required to precisely transfer the pattern formed on the reticule onto the object to be exposed with a given magnification. For that reason, it is important to use the projection optical system that offers high imaging performance and suppresses aberration. Particularly, in recent years, due to a growing demand for further miniaturized semiconductors, even the small aberration of the optical system greatly affects the transfer pattern. For that reason, there is a demand for measuring the optical performance (for example, wavefront aberration, Zernike coefficient) of the projection optical system with high precision in a state where the projection optical system is mounted on the exposure apparatus main body. Also, it is important to simplify and speed up the measurement and to reduce the costs from the viewpoints of enhancing the productivity and cost efficiency.
As a method of measuring the optical performance of the projection optical system, there is a method in which a mask pattern is actually etched into a wafer, and a resultant resist image is observed by a scanning electron microscope (SEM) or the like for inspection. However, this method has a problem in that the inspection takes time by performing exposure, development, and the like, and the reproducibility of the inspection suffers because of the difficulty of the operation of the SEM and an error caused in a process of resist coating or developing. In order to solve the above problem, there has been proposed that the optical performance of the projection optical system is measured by using a point diffraction interferometer (PDI) having a pin hole for forming an ideal spherical wave. Moreover, it has been proposed to measure the optical performance of the projection optical system by using a shearing interferometer (or Talbot interferometer). In recent years, it has been also proposed to measure the optical performance of the projection optical system by using a line diffraction interferometer (LDI) having a slit for forming an ideal cylindrical wave or an ideal elliptical wave (for example, refer to Japanese Patent Application Laid-Open No. S57-064139, Japanese Patent Application Laid-Open No. 2000-146705, and Japanese Patent Application Laid-Open No. 2000-097666). In addition, the applicants of the present invention have proposed an exposure apparatus equipped with an interferometer in Japanese Patent Application Laid-Open No. 2000-277411 and Japanese Patent Application Laid-Open No. 2000-277412.
It should be noted that, when exposure light has high coherency, light beams of the exposure light that have passed through the pattern on the mask interfere with one another. As a result, the pattern cannot be accurately transferred onto a wafer. For that reason, the exposure apparatus usually makes the exposure light incoherent in an illumination optical system. However, when the exposure apparatus is equipped with an interferometer, the interferometer must utilize an exposure light having low coherency, which leads to a deterioration of the visibility of interference fringes. As a result, there arises a problem in that a precision in the measurement of the wavefront aberration is degraded. In this specification, the visibility V is defined by the following formula,V=(Imax−Imin)/(Imax+Imin),where Imax represents the maximum amount of light of the interference fringes, and Imin represents the minimum amount of light of the interference fringes.